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If it’s never been treated then happy days.
You might be able to bring it up nicely with some deck specific cleaner and a scrubbing brush. Then finish it how you want or just leave it raw if you don’t mind it greying.
Pressure washing will clean it back well but it’s a very messy job and you can bugger the wood surface up if you get to close so it won’t take a treatment and will need sanding anyway. A patio attachment is mandatory if you don’t know what you are doing.
Our deck had what was either a paint or a stain on it which failed locally. Over time I’ve been removing it all with various horrible chemical and mechanical methods. It’s a big deck too.
5 years old, never been oiled as far as I'm aware.
Which is what exactly? Sanding?
Any help would be appreciated